Add updated documentation for debugging data stoppage alerts.

tools/perf/docs/triaging_data_stoppage_alerts.md

+# Triaging Data Stoppage Alerts
+
+## What is a data stoppage alert?
+A data stoppage alert is a new type of alert on the perf dashboard. Instead of a
+performance regression, it indicates that the dashboard is no longer receiving
+data for the given monitored test. A bug created from a data stoppage alert has
+a subject starting with **“No data received for…”**.
+
+## How to triage data stoppage alerts
+
+### Check if the alert is recovered.
+Look at the graph and see if there are new points, if so, mark the alert
+**ignored**.
+
+### File a bug
+Use the triage dialog to file a bug about the failure, and track your
+investigation. Cc the owner of the benchmark from
+[go/chrome-benchmarks](http://goto.google.com/chrome-benchmarks).
+
+### Get the logs
+Each alert has a debug button at the right-hand side of the table. It tries to
+automatically find the last successful build and the first failed build. To get
+the logs:
+  * First try the *"Logs"* link from *"Next revison built"* (this should be the
+    first failed revision). Sometimes this can't be generated properly, so it
+    may not work.
+  * Next try the *"Buildbot status page"* link from *"Next revision built"*.
+    This should take you to the next build. **If this page 404s, it's possible
+    the builder was taken down.** Check the waterfall.
+
+Once you have the logs, put the link in the bug and also paste relevant snippets
+about the failure (error logs) in the bug.
+
+### Check for suspicious changes.
+It has a link to *"View commit log from rXXX to rYYY"*, click the link to view
+CLs in the range. Look through the range for test disables, telemetry/catapult
+changes, and changes to the code under test. If you see a CL that looks like a
+likely culprit, cc the author in the bug.
+
+### Kick off a bisect.
+If the test is failing on the *"Next revision built"*, bisect may be able to
+narrow down the culprit. Go to the graph, click a data point, and click the
+bisect button in the tooltip. **You'll need to change the values for return_code
+bisect as follows**:
+  * **Bug ID**: Make sure to fill in the ID of the bug you just filed.
+    Otherwise the bisect will not update it.
+  * **Earlier revision**: Change this to the *"Last revision uploaded"* from the
+    debug button dialog.
+  * **Later revision**: Change this to the *"Next revision built"* from the
+    debug button dialog.
+  * **Bisect mode**: Change this to **return_code**